1. Technical Field of the Invention
The invention relates generally to communication systems; and, more particularly, it relates to synchronous communication systems.
2. Description of Related Art
Cable modem communication systems have been under continual development for the last several years. There has been development to try to provide for improvements in the manner in which communications between one or more Cable Modems (CMs) and a Cable Modem Termination System (CMTS) is performed. A focus of the development within these cable modem communication systems has been to improve the manner in which broadband network access to an ever increasing number of CMs within the cable modem communication system.
Cable modem communication systems are realized when a cable company offers network access, oftentimes the Internet, access over the cable that is provisioned to provide cable television services to a subscriber. This way, the Internet information can use the same cables because the cable modem communication system provides downstream data, sent using the CMTS from the Internet to an individual computer having CM functionality, into a communication channel having a 6 MHz capacity. The data provided in the reverse direction within the cable modem communication system is typically referred to as upstream data, namely, information that is sent from an individual (e.g., from a CM) back to the Internet via the CMTS. The transmission of upstream data typically requires even less of the cable's available bandwidth. Some estimates say only 2 MHz are required for the upstream data transmission; this is based on the assumption is that most people download far more information than they upload to/from the Internet.
Putting both upstream and downstream data on the cable television system requires two types of equipment: a CM on the customer end and the CMTS at the cable provider's end. Between these two types of equipment, all the computer networking, security and management of Internet access over cable television is put into place. This intervening region may be generically referred to as a CM network segment, and a variety of problems can occur to signals sent across this CM network segment.
One particular problem is the effects of multi-path. This multi-path degradation may be viewed as being an attenuated, shifted (delayed) replica of a signal that is transmitted within the cable that resides in the CM network segment. The multi-path degradation, along with other undesirable signal modifying effects, can significantly reduce the ability to maximize the full bandwidth potential of the cable modem communication system. This may result in slower service to subscribers that use CMs, which may lead these users to seek Internet access elsewhere. There is continually an ever-increasing desire to maximize the throughput of communication systems, and cable modem communication systems are no exception. The undesirable effects of multi-path, along with other interference generating effects within channel segments within the cable modem communication system, produce significantly degraded performance of the overall system.
Another significant problem may arise from the physical properties of the devices employed at the CM and CMTS. One such problem arises from the use of analog Voltage Controlled Oscillators (VCOs) within the CM and CMTS. In most prior art approaches, analog VCOs and voltage controlled crystal oscillators (VCXOs) are used to lock to a reference. The inherent analog implementation of the locking of the CM upstream to the downstream symbol clock of the CMTS, being implemented typically using phase locked loops (PLLs), introduces a number of deleterious effects. For example, the analog design may undesirably introduce a multitude of problems, including pickup of circuit board noise, lack of precise control of parameters, extra and increased parts cost, and other deficiencies as well. The inherent nature of these analog devices will introduce some noise into the system. In addition, the emitted frequency from these devices may vary over temperature, humidity, and other environmental variations.
A number of operational degradations may result when upstream and downstream transmissions are not properly synched. For example, in the presence of carrier frequency offset between the CM and the CMTS, a modulation's constellation may actually spin when trying to sample received information. In addition, when there is phase error in a received signal, then a modulation's constellation may be rotated by some degree. In the presence of gain error in a received signal, then the constellation points of a modulation may be further in and/or closer out that they should be based on the placement of points within the constellation. For at least these reasons, there is a need to ensure that the upstream and downstream transmissions are properly locked.
Moreover, within cable modem communication systems that operate using S-CDMA (Synchronous Code Division Multiple Access) modulation, the synchronous operation of the various elements within the cable modem communication system is imperative and required by the Data Over Cable Service Interface Specifications (DOCSIS), draft 2.0. Presently, there does not yet exist in the art a sufficient solution to ensure the synchronous operation of the various devices within the cable modem communication system as required by DOCSIS.